1. Field of the Invention
The present invention relates to a bi-directional optical amplifier optically amplifying a bi-directionally propagating signal light.
2. Description of the Related Art
A rare-earth element added optical fiber, made by adding a rare-earth element such as the Erbium(Er) to silica (SiO.sub.2) optical fiber, is well-known as an optical amplification element, which optically amplifies a damped bi-directional signal light and transmits it a long distance. In recent years, the optical amplifier with the optical amplification element has been used in place of the amplifier with a function of making a photoelectric conversion.
In the optical communication systems, a bi-directional optical communication or a bi-directional reception and transmission of a signal light are both made. In the bi-directional optical communication, in order to more efficiently use the optical transmission line, the bi-directional optical communication system is available where identical or different wave-lengths of signal lights are bi-directionally propagated along an optical fiber.
When an optical amplifier with the above optical amplification element is applied to the bi-directional optical communication system in order to amplify optically, a bi-directional optical amplifier is necessary. The conventional bi-directional optical amplifiers have incorporated a technology, for example, the one disclosed in "Japanese Patent Application Laid-open No. Hei-9-18417" or "Japanese Patent Application Laid-open No. Hei-4-62528". The former (Japanese Patent Application Laid-open No. Hei-9-18417) is configured in such a way that EDFs (Erbium Doped Fibers) are connected to four ports in an optical circulator, respectively. On the other hand, the latter(Japanese Patent Application Laid-open No. Hei-4-62528) is configured in such a manner that an optical device, other than the rare-earth element added optical fibers, is connected to the input/output ports of an optical circulator.
However, both the above configurations need a reflection point upon which a signal light can be totally reflected. In addition, an optical filter is required in order to prevent a possible emission of spontaneous emission light at an optical input/output end. Accordingly, its configuration becomes complex, causing an increase of transmission losses.